1. Field of Invention
This invention relates generally to chromophores having two-photon or other multi-photon absorptivity.
2. Related Art
Organic molecules that absorb two or more photons simultaneously have wide application in a variety of technologies involving such subjects as optical data storage, 3-D microfabrication techniques, frequency upconverted lasing, optical power limiting, photodynamic therapy, initiators of polymerization reactions, and multi-photon fluorescence microscopy for biological imaging. Two features of the two-photon absorption process make these applications feasible. The first is the quadratic dependence of two-photon absorption on the intensity of the incident radiation. This allows for three dimensional spatial resolution. The second feature is the absence of single-photon absorption, which allows an incident light beam to penetrate deeper into a material than would be possible with single-photon approaches.
Two-photon or other multi-photon absorbing molecules have been designed based on conjugated pi-electron systems with donating groups at each end of the pi-electron system providing charge-transfer properties. For example, U.S. Pat. Nos. 6,267,913 and 6,608,228, both incorporated by reference herein, describe two-photon absorbing chromophores having electron donors such as amino or alkoxy groups attached to a bridge of pi-conjugated bonds. The absorption of two or more photons by such molecules can trigger chemical and physical changes that make these substances useful for two-photon applications. As an example, in photo-polymerization reactions, two-photon absorption by a chromophore leads to the production of reactive starting species such as free radicals, cations or anions that initiate the polymerization process.